Touch-control interface is now widely used in mobile devices, such as cell phones, personal digital assistants (PDAs), etc. Generally, a touch-controlled mobile device includes a lock screen to prevent unauthorized or unintentional access to the device, and needs to undergo a two-stage unlocking process when resuming normal operation from sleep mode. A first operation needs to be performed on an unlock screen displayed on the device (e.g. slide to unlock) to prevent unintentional touch s during sleep mode. Next, a second unlock operation (e.g. entering a password) needs to be performed to prevent unauthorized access to the device.
However, such two-stage unlocking process can be cumbersome. As a result, many users are discouraged from configuring a second-stage unlock operation for their mobile devices, thus posing a potential security loophole. Therefore, searching for a simple yet secure unlock method for mobile devices has become a common goal for the industry.